Technical Field
The disclosure relates generally to manifolds for stack assemblies, and more particularly to external manifolds for fuel cell stack assemblies. More particularly still, the disclosure relates to such external manifolds for fuel cell stack assemblies that may be located in leak-sensitive environments.
Description of the Related Art
In cell stack assemblies, it is known to use internal reactant and coolant manifolds as well as external reactant and coolant manifolds. Internal manifolds generally comprise passageways made within the various plates that constitute the cells of the cell stack assemblies. This renders the plates themselves more expensive than plates which are fabricated for use with external manifolds. Internal manifolds have potential leakage paths between the plates of every cell, the leakage thereby being “overboard” to the external environment. While external manifolds may also leak gasses to the external environment, the avoidance of such leaks is more easily accomplished. The operating location of the cell stack assemblies and associated manifolds is often determinative of the level of gas (typically reactant) leakage that can be tolerated to the environment. Indeed, in some instances, relatively greater leakage can be tolerated if the leakage region can be purged with air or the like. Typically, various closed spaces, as for example vehicular environments containing humans, are relatively more leak-sensitive, i.e., less leak-tolerant, and the ability to purge the leakage may be very limited or impractical.
External manifolds consist of manifold shells, manifold seal gaskets, and a mechanical loading or restraint system to hold the manifolds in compression, tightly against the edges of the cell stack.
A fuel cell module 10 of U.S. Pat. No. 4,345,009, shown in FIG. 1 and in cross section in FIG. 2, is one example of the external manifold and containment systems known in the art. The lower right corner of FIG. 2 is the corner of the module 10 pointing toward the viewer in FIG. 1. The module 10 includes a stack 12 of fuel cells 14. As shown in FIG. 3, each fuel cell 14 comprises a gas porous anode electrode 16 and a gas-porous cathode electrode 18 spaced apart with a layer 20, such as a liquid electrolyte-retaining matrix or a proton exchange membrane (PEM), disposed there between. Each electrode 16, 18 includes a very thin catalyst layer 19, 21, respectively on the surfaces thereof adjacent the layer 20. An electrically conductive, gas impervious plate 22 may separate adjacent fuel cells in the stack 12. Each fuel cell in the stack may include one separator plate 22 such that the phrase “fuel cell” will encompass a repeating unit of the stack which includes one separator plate. The fuel cells of this exemplary embodiment may be the same as shown in U.S. Pat. No. 4,115,627 in which the electrolyte is phosphoric acid. However, fuel cell stacks with proton exchange membrane electrolytes, as in U.S. Pat. No. 6,024,848, have similar needs with respect to manifolds and their integrity.
In this embodiment, every third fuel cell 14′ (FIG. 3) includes a coolant carrying layer 24 disposed between the electrode 16 and the separator plate 22. Passing in-plane through this layer 24 are coolant carrying passages 26. The coolant flow through these passages carries away the heat generated by the fuel cells. The number of coolant layers 24 and passages 26 required by a stack is dictated by a variety of factors which are not relevant here. Although the coolant passages 26 are shown as extending to the surface 32 for clarity, in an actual fuel cell stack they would only do so in regions adjacent to two corners of the cell. The stack 14 is completed by top and bottom flat graphite current collector blocks 27, 28, respectively, bonded to the separator plates 22 at each end of the stack, and pressure plates 66, 68.
As shown in the drawing, the outer edges 29 of the stack components 16, 18, 20, 22, 24, 27 and 28 form four outwardly facing, approximately planar surfaces which are the external surfaces of the stack 12. Portions of two of these surfaces 30, 32 are shown in FIG. 3. Each of the four surfaces is substantially completely covered by a reactant gas manifold. An air or oxygen gas inlet manifold 34 covers the surface 30 while a fuel or hydrogen gas inlet manifold 36 covers the surface 32. The opposing surfaces are covered by an air outlet manifold 38 and a fuel outlet manifold 40 (FIG. 2).
The manifolding arrangement just described incorporates an outlet manifold on each side of the stack opposite an inlet manifold. However, as shown in in U.S. Pat. No. 3,994,748, a fuel manifold covering one surface of the stack may be divided into two compartments to serve as both the inlet and the outlet manifold, while the manifold on the opposite surface of the stack serves as a mixing manifold; the same configuration may be used for the air.
The anode electrode 16 and the cathode electrode 18 both comprise relatively thick substrates with ribs formed on one side thereof defining reactant gas channels 42, 44, respectively. The fuel gas channels 42 carry hydrogen or a hydrogen-rich gas across the cells from the fuel inlet manifold 36 to the fuel outlet manifold 40. The air channels 44 carry air across the cells from the air inlet manifold 34 to air outlet manifold 38. The flat surface of each substrate, which is opposite to the surface having the ribs (and thus the gas channels), has a layer 19, 21 of catalyst disposed thereon.
The graphite blocks 27, 28 have the same outer dimensions as the other stack components, and their outwardly facing surfaces (two of which, 50 and 52, can be seen in FIG. 3) provide smooth sealing surfaces for the top and bottom sealing flanges 54, 56 of each manifold. A thick block at one end of the stack is required to accommodate the possible differences in stack height (or length, depending on stack orientation) which could result from the buildup of the very small tolerances in the thickness of the many hundreds of components in the stack 12. For example, a stack of 400 cells each having a thickness of about 0.64 cm (0.25 inch) with a tolerance of 0.01 cm (±0.004 inch) could have an overall height of anywhere from 250 to 258 cm (98.4 to 101.6 inches). The manifolds, on the other hand, have a fixed height (length) A large block thickness is thus required to ensure that both the top and bottom flanges 54, 56 are located somewhere on the smooth sealing surfaces of the blocks 27, 28 after the desired compressive force has been applied to the stack, as hereinafter explained.
As best shown in FIG. 2, side flanges 58 seal against the vertically extending external surfaces of the stack 12 near the corners of the stack which do not have reactant gas channels. A sealing material, such as a porous polytetrafluoroethylene, is disposed between the manifold flanges 54, 58 and the surfaces of the stack. Steel bands 60 (FIGS. 1 and 2) surround the stack manifolds, and hold them in sealing relationship with the stack and graphite blocks. Fasteners 62 connecting the ends of each band permit tightening the bands to the extent necessary to ensure adequate sealing.
To obtain good electrical, thermal, and sealing contact between the various components of the fuel cells and the stack 12, the module 10 includes a constraint system 64. In this exemplary embodiment, the constraint system 64 comprises inflexible top and bottom steel end or pressure plates 66, 68, respectively, and tie rods 70 connecting the plates. The plates 66, 68 rest flat against the graphite blocks 27, 28, respectively. In assembling a module 10, the pressure plates 66, 68, the blocks 48, 49, and the various stack components are arranged one atop (or adjacent, depending on orientation) the other in proper sequence. This assembly is hydraulically loaded whereupon a preselected axial (i.e., perpendicular to the plane of the cells) load is applied to the plates 66, 68 to compress the stack 12. The tie bolts 70 are then tightened down to an extent that, when the assembly is removed from the press, the compressive force on the stack 12 is of approximately the desired magnitude. The manifolds 34, 36, 38, and 40 are then positioned against the sides of the stack and secured by the bands 60.
Since the constraint system 64 and the manifolds 34, 36, 38, and 40 are made from similar materials (carbon steel), they have the same or approximately the same coefficient of thermal expansion. Therefore, when the stacks heat up during operation, these items expand to approximately the same extent. Although the stack 12 has a lower coefficient of thermal expansion, as the plates 66, 68 move apart the elasticity or spring rate of the compressed stack 12 results in the height of the stack 12 increasing by the same rate with an accompanying loss in axial load. Thus, there is virtually no relative movement between the graphite blocks 27, 28 and their respective manifold sealing flanges 54, 56 during thermal expansion. Likewise, there is relatively little motion between the stack external surfaces, such as 30 and 32, and the vertical manifold sealing flanges 58. Once steady state is reached the constraint system 64 holds the stack height constant.
The external manifold system described with respect to FIGS. 1-3 presents difficulty in assuring the avoidance of leakage overboard to the external environment. Although the four outwardly facing surfaces formed by the sides of the assembled stack components 16, 18, 20, 22, 24, 27 and 28 of that U.S. Pat. No. 6,764,787 were characterized above as being “approximately planar”, in fact they are typically somewhat irregular due to minor inconsistencies in the sizes and/or alignments of those assembled stack components. This may be better understood with reference to U.S. Pat. No. 6,660,422, which describes and depicts the irregularity of such surfaces. These irregularities in those outwardly facing surfaces of the stack further complicate efforts to obtain a good seal between the manifolds and at least those surfaces of the stack. The possible leakage of fuels, such as hydrogen, in prior art fuel cell assembly designs may be particularly undesirable in closed spaces, as for example in human-occupied vehicles and the like, where the elimination or dilution of such leaked fuel is not possible or is inadequate.
However, the use of external manifolds may be advantageous for various reasons. External manifolds can be lower in product cost to manufacture, and provide a protective shield around the fuel cells top protect them from physical damage and contamination. External manifolds can also provide an electrical short protection when combined with dielectric seal materials, plastic coated metals, and/or made from plastic materials. They can provide structural support for the cell stack. External manifolds can also provide the flexibility of integrating reactant flow and coolant distribution features, such as baffles and/or chambers to allow the fuel cell stack to operate with better reactant and coolant usage and to make the fuel cell stack more durable.